Panel reverse bias behaviour / blocking diode

bdl

Ben
I'd wondered what would happen when a panel directly connected to a battery went dark, would the panel draw power from the battery?

Turns out, no: I connected a voltage and current-controlled power supply to a set of three parallel Victron SPP041751200 175W panels, at night, with the current limit set to 1A and swept the applied voltage from 13 to 21V with the following results:
1601645919159.png
I also did the same with a single JA Solar JAP60S01-275/SC, with exactly the same shape curve but higher voltages (the JA Solar panel has a Voc of 38.4V).

Note at a voltage of 14V (max 4S LiFePO4 battery) there is only around 13mA (0.2W) power loss to the panel. I wasn't game to go above 21V/1A, but I presume the panel would exhibit the usual diode avalanche and become mostly a resistive load (something I believe Dacian / electrodacus has experimented with in terms of using a panel as a heater).

I know that some setups use a blocking diode to prevent any reverse current through the panel, I can't particularly think of a situation where that would be necessary.
 
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